1. Field of the Invention
The use of the present invention relates to an improved method for bonding polystyrene foam details together using microwave curing phenol-formaldehyde compounds to produce lost foam molds suitable for casting metallic details.
2. Description of Related Art
The automotive industry employs molds for casting aluminum engine parts (such as heads and blocks) by bonding "lost foam" polystyrene foam details with a hot melt (thermoplastic) glue using an automated adhesive application work station. This work station consists of a rotating turntable fitted with hot platens that mirror the configuration of the polystyrene parts to be bonded. The hot melt adhesive, in the presence of the hot platens, becomes sufficiently fluid to be able to transfer the glue to the bonding surfaces of the polystyrene parts via contact. The parts are then mated prior to the glue cooling in order to achieve an adequate bond.
This process allows the production of approximately five assemblies per minute. However, not only is this process considered to be too slow for the amount of production required, it is inherently messy. The messiness results from the glue flowing into unwanted features on the foam, as well as on and about the bonding work station itself. In addition, any change in foam configuration necessitates a capital expenditure to fabricate the corresponding platen.
U.S. Pat. No. 4,035,216, "Method for Bonding Expanded Polymeric Parts", describes a process for bonding polystyrene foam lost foam patterns together using a water/detergent mixture as the electromagnetic susceptor compound with a microwave energy source. The detergent can make it exceedingly difficult to perform secondary bonding or coating operations on the joined polystyrene foam due to the release layer that has been introduced on these surfaces. Moreover, the residual water from this "solvent" bonding process results in porosity in the resulting aluminum castings produced from these lost foam polystyrene foam molds. This porosity can result in decreases in mechanical strength of the castings, as well as marring surface finish.
U.S. Pat. No. 4,253,898, "Bonding Composition and Microwave Process for Bonding Together Plastic Components", describes a vinyl plastisol composition for microwave bonding of plastic parts. This process is dependent upon the melting of the plastic, which can distort the joined device. This can result in the lost foam details transferring incorrect dimensions to the final metal casting.
U.S. Pat. No. 4,018,642, "Microwave Curing of Alkaline Phenolic Resins in Wood-Resin Compositions", describes the use of phenolic resins for microwave bonding of plywood. No inference with respect to the utility of bonding polystyrene foam lost foam details with this process is mentioned.
Thus, a need exists to provide an improved method for bonding polystyrene assemblies that avoids or minimizes the above mentioned disadvantages.